Optical receiver, light signal receiving method, and data reproduction device

ABSTRACT

A coherent receiver performing coherent detection on polarization multiplex light into which first polarization light and second polarization light are multiplexed, and splitting the polarization multiplex light into the first polarization light and the second polarization light, an adaptive equalizer compensating the waveform distortion of a signal superimposed onto the first polarization light by using a first FIR filter, compensating the waveform distortion of a signal superimposed onto the second polarization light by using a second FIR filter, and by decoding each of the signals whose waveform distortion has been compensated, generating their respective decoded data, an error ratio calculator calculating the bit error ratio of each decoded data generated by the adaptive equalizer, a margin calculator calculating a margin from the bit error ratio of an error correction limit in each bit error ratio calculated by the error ratio calculator, and a tap number controller setting up the numbers of taps of the first and second FIR filters by referring to the respective margins calculated by the margin calculator are included.

CROSS REFERENCE TO RELATED APPLICATION

This application is a Continuation of PCT International Application No.PCT/JP2018/030985, filed on Aug. 22, 2018, which is hereby expresslyincorporated by reference into the present application.

TECHNICAL FIELD

The present disclosure relates to an optical receiver, a light signalreceiving method, and a data reproduction device that set up the numberof taps of a finite impulse response filter.

BACKGROUND ART

In recent years, in optical communication systems, a digital coherenttechnique in which digital signal processing and coherent detection arecombined is used in order to implement long distance transmission oflight signals and transmission of large-volume light signals.

In digital coherent techniques, polarization multiplex light istransmitted using, for example, dual-polarization quadrature phase shiftkeying (DP-QPSK).

Polarization multiplex light may have a polarization variation under theinfluence of turbulence or the like when transmitted through atransmission line.

It is known that a polarization variation in polarization multiplexlight affects the performance of improving the bit error ratio (BER) ofreception in an optical receiver.

In Patent Literature 1 below, a digital coherent receiver that candemodulate a received signal even when a variation occurs in the stateof polarization (SOP) in a transmission line is disclosed. The digitalcoherent receiver disclosed in Patent Literature 1 performs adaptivecontrol on a step size parameter by which the tap coefficient of thefinite impulse response (FIR) filter of an adaptive equalizer ismultiplied when updating the tap coefficient, in accordance with thespeed of the variation of SOP.

CITATION LIST Patent Literature

Patent Literature 1: JP 2013-223128 A

SUMMARY OF INVENTION Technical Problem

The digital coherent receiver disclosed in Patent Literature 1 cansuppress the degradation in the transmission quality caused by theinfluence of the polarization variation when the polarization variationis a gradual one varying in several seconds.

However, a problem is that when the polarization variation is a rapidone varying in a time of several tens of microseconds, it is difficultto suppress the degradation in the transmission quality caused by theinfluence of the polarization variation only by performing the adaptivecontrol on the step size parameter.

The present disclosure is made in order to solve the above-mentionedproblem, and it is therefore an object of the present disclosure toprovide an optical receiver, a light signal receiving method, and a datareproduction device capable of, even when a polarization variation is arapid one varying in a time of several tens of microseconds, suppressingthe degradation in the transmission quality caused by the influence ofthe polarization variation.

Solution to Problem

An optical receiver according to the present disclosure includes: anoptical receiver comprising: a coherent receiver performing coherentdetection on polarization multiplex light into which first polarizationlight and second polarization light are multiplexed, and splitting thepolarization multiplex light into the first polarization light and thesecond polarization light; an adaptive equalizer compensating thewaveform distortion of a signal superimposed onto the first polarizationlight by using a first finite impulse response filter, compensating thewaveform distortion of a signal superimposed onto the secondpolarization light by using a second finite impulse response filter, andby decoding each of the signals whose waveform distortion has beencompensated, generating their respective decoded data; an error ratiocalculator calculating the bit error ratio of each decoded datagenerated by the adaptive equalizer; a margin calculator calculating amargin from the bit error ratio of an error correction limit in each biterror ratio calculated by the error ratio calculator; and a tap numbercontroller for setting up the numbers of taps of the first and secondfinite impulse response filters by referring to the respective marginscalculated by the margin calculator.

Advantageous Effects of Invention

According to the present disclosure, the optical receiver is configuredin such a way as to include the tap number controller for setting up thenumbers of taps of the first and second finite impulse response filterson the basis of the respective margins calculated by the margincalculator. Therefore, the optical receiver according to the presentdisclosure can suppress the degradation in the transmission qualitywhich is caused by the influence of the polarization variation even inthe case in which the polarization variation is a rapid one varying in atime of several tens of microseconds.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram showing a data reproduction deviceincluding an optical receiver according to Embodiment 1;

FIG. 2 is a schematic diagram showing a digital signal processor 14 ofthe optical receiver 1;

FIG. 3 is a hardware block diagram showing the hardware of the digitalsignal processor 14;

FIG. 4 is a hardware block diagram of a computer in the case in whichthe digital signal processor 14 is implemented by software, firmware, orthe like;

FIG. 5 is a flowchart showing a part of a light signal receiving methodwhich is a processing procedure in the case in which the digital signalprocessor 14 is implemented by software, firmware, or the like;

FIG. 6 is a flowchart showing a process of setting up the number of tapsin a tap number controller 25;

FIG. 7 is an explanatory drawing showing a relation between the numberof taps of an FIR filter and a margin in a bit error ratio of decodeddata; and

FIG. 8 is an explanatory drawing showing a relation between the numberof taps of an FIR filter and a margin in a bit error ratio of decodeddata.

DESCRIPTION OF EMBODIMENTS

Hereinafter, in order to explain the present disclosure in greaterdetail, embodiments of the present disclosure will be described withreference to the accompanying drawings.

Embodiment 1

FIG. 1 is a schematic diagram showing a data reproduction deviceincluding an optical receiver 1 according to Embodiment 1.

FIG. 2 is a schematic diagram showing a digital signal processor 14 ofthe optical receiver 1.

FIG. 3 is a hardware block diagram showing the hardware of the digitalsignal processor 14.

In FIGS. 1 to 3, the optical receiver 1 includes a light source 11, acoherent receiver 12, an analog to digital converter 13, and a digitalsignal processor 14.

The optical receiver 1 receives polarization multiplex light into whichfirst polarization light and second polarization light are multiplexed,and decodes a signal superimposed onto each of the first and secondpolarization light rays, thereby generating their respective decodeddata.

In Embodiment 1, for example, it is assumed that the first polarizationlight is horizontally polarized light, and the second polarization lightis vertically polarized light.

It is assumed that a signal (referred to as a “first transmissionsignal” hereinafter) which is information to be transmitted issuperimposed onto the horizontally polarized light, and a signal(referred to as a “second transmission signal” hereinafter) which isinformation to be transmitted is superimposed onto the verticallypolarized light. As each information to be transmitted, imageinformation or measurement information can be considered.

Further, it is assumed that the first and second transmission signalsare modulated signals of DP-QPSK.

The light source 11 generates local oscillation light having the samewavelength as the polarization multiplex light, and outputs the localoscillation light to the coherent receiver 12.

The coherent receiver 12 is implemented by, for example, a polarizationbeam splitter, a 90-degree optical hybrid, and a photo diode.

The coherent receiver 12 receives the polarization multiplex light, andperforms coherent detection on the polarization multiplex light by usingthe local oscillation light outputted from the light source 11.

The coherent receiver 12 splits the polarization multiplex light intothe horizontally polarized light and the vertically polarized light, andconverts each of the horizontally and vertically polarized light raysinto an electric signal.

Hereinafter, the electric signal into which the horizontally polarizedlight is converted is referred to as the “horizontal polarizationsignal”, and the electric signal into which the vertically polarizedlight is converted is referred to as the “vertical polarization signal.”

The coherent receiver 12 outputs each of the horizontal and verticalpolarization signals to the analog to digital converter 13.

The analog to digital converter 13 converts the horizontal polarizationsignal outputted from the coherent receiver 12 from an analog signalinto a digital signal, and outputs the digital signal, as a digitalhorizontal polarization signal, to the digital signal processor 14.

The analog to digital converter 13 converts the vertical polarizationsignal outputted from the coherent receiver 12 from an analog signalinto a digital signal, and outputs the digital signal, as a digitalvertical polarization signal, to the digital signal processor 14.

The digital signal processor 14 includes an adaptive equalizer 21, anerror corrector 22, an error ratio calculator 23, a margin calculator24, and a tap number controller 25.

The digital signal processor 14 compensates for the waveform distortionof the first transmission signal superimposed onto the digitalhorizontal polarization signal, and decodes the first transmissionsignal after the waveform distortion compensation, thereby generatingdecoded data (referred to as “first decoded data” hereinafter).

The digital signal processor 14 compensates for the waveform distortionof the second transmission signal superimposed onto the digital verticalpolarization signal, and decodes the second transmission signal afterthe waveform distortion compensation, thereby generating decoded data(referred to as “second decoded data” hereinafter).

The adaptive equalizer 21 is implemented by, for example, an adaptiveequalization circuit 41 shown in FIG. 3.

The adaptive equalizer 21 includes a first finite impulse responsefilter and a second finite impulse response filter. Hereinafter, thefinite impulse response filters are referred as the FIRs.

The adaptive equalizer 21 compensates for the waveform distortion of thefirst transmission signal superimposed onto the digital horizontalpolarization signal by using the first FIR filter, and decodes the firsttransmission signal after the waveform distortion compensation, therebygenerating first decoded data.

The adaptive equalizer 21 compensates for the waveform distortion of thesecond transmission signal superimposed onto the digital verticalpolarization signal by using the second FIR filter, and decodes thesecond transmission signal after the waveform distortion compensation,thereby generating second decoded data.

The adaptive equalizer 21 outputs each of the first and second pieces ofdecoded data to the error corrector 22.

The error corrector 22 is implemented by, for example, an errorcorrection circuit 42 shown in FIG. 3.

The error corrector 22 performs error correction to the first decodeddata generated by the adaptive equalizer 21, and calculates the numberof error corrections in the first decoded data.

The error corrector 22 performs error corrections to the second decodeddata generated by the adaptive equalizer 21, and calculates the numberof error corrections in the second decoded data.

The error corrector 22 outputs each of the first and second pieces ofdecoded data after the error correction to the data reproducer 30, andoutputs each of the numbers of error corrections in the first and secondpieces of decoded data to the error ratio calculator 23.

The error ratio calculator 23 is implemented by, for example, an errorratio calculation circuit 43 shown in FIG. 3.

The error ratio calculator 23 calculates a bit error ratio of the firstdecoded data after the error correction outputted from the errorcorrector 22, and calculates a bit error ratio of the second decodeddata after the error correction outputted from the error corrector 22.

For example, the error ratio calculator 23 calculates the bit errorratio of the first decoded data from the number of error corrections inthe first decoded data, the number of error corrections being calculatedby the error corrector 22, and calculates the bit error ratio of thesecond decoded data from the number of error corrections in the seconddecoded data, the number of error corrections being calculated by theerror corrector 22.

The error ratio calculator 23 outputs each of the bit error ratios ofthe first and second pieces of decoded data to the margin calculator 24.

The error ratio calculator 23 should just be able to calculate the biterror ratios of the first and second pieces of decoded data, and thecalculation is not limited to the one of the bit error ratios of thefirst and second pieces of decoded data from the numbers of errorcorrections to the first and second pieces of decoded data. Therefore,the error ratio calculator 23 may receive the first and second pieces ofdecoded data which are outputted from the adaptive equalizer 21 andcalculate the bit error ratios of the first and second pieces of decodeddata. In this case, the error corrector 22 is unnecessary for thecalculation of the bit error ratios.

The margin calculator 24 is implemented by, for example, a margincalculation circuit 44 shown in FIG. 3.

The margin calculator 24 calculates a margin (referred to as a “firstmargin” hereinafter) from the bit error ratio of the error correctionlimit in the bit error ratio of the first decoded data calculated by theerror ratio calculator 23.

The margin calculator 24 calculates a margin (referred to as a “secondmargin” hereinafter) from the bit error ratio of the error correctionlimit in the bit error ratio of the second decoded data calculated bythe error ratio calculator 23.

The bit error ratio of the error correction limit is a value which isdetermined in the error corrector 22 depending on the configuration forperforming error correction, and the bit error ratio of the errorcorrection limit is known in the margin calculator 24.

The margin calculator 24 outputs each of the first and second margins tothe tap number controller 25.

The tap number controller 25 is implemented by, for example, a tapnumber setting circuit 45 shown in FIG. 3.

The tap number controller 25 sets up the number of taps of the first FIRfilter which the adaptive equalizer 21 uses, on the basis of the firstmargin calculated by the margin calculator 24.

The tap number controller 25 sets up the number of taps of the secondFIR filter which the adaptive equalizer 21 uses, on the basis of thesecond margin calculated by the margin calculator 24.

The data reproducer 30 records each of the first and second pieces ofdecoded data after the error correction which are outputted from theerror corrector 22.

The data reproducer 30 generates the first and second pieces of decodeddata recorded. For example, when the first and second pieces of decodeddata are pieces of image data, the data reproducer 30 draws an image byreproducing the first and second pieces of decoded data.

In FIG. 2, it is assumed that each of the following components: theadaptive equalizer 21, the error corrector 22, the error ratiocalculator 23, the margin calculator 24, and the tap number controller25, which are the components of the digital signal processor 14, isimplemented by hardware for exclusive use as shown in FIG. 3. Morespecifically, it is assumed that the digital signal processor 14 isimplemented by the adaptive equalization circuit 41, the errorcorrection circuit 42, the error ratio calculation circuit 43, themargin calculation circuit 44, and the tap number setting circuit 45.

Here, each of the following components: the adaptive equalizationcircuit 41, the error correction circuit 42, the error ratio calculationcircuit 43, the margin calculation circuit 44, and the tap numbersetting circuit 45 is, for example, a single circuit, a compositecircuit, a programmable processor, a parallel programmable processor, anapplication specific integrated circuit (ASIC), a field-programmablegate array (FPGA), or a combination of these circuits.

The components of the digital signal processor 14 are not limited toones each implemented by hardware for exclusive use, and the digitalsignal processor 14 may be implemented by software, firmware, or acombination of software and firmware.

The software or the firmware is stored as a program in a memory of acomputer. The computer refers to hardware that executes a program, andis, for example, a central processing unit (CPU), a central processingdevice, a processing device, an arithmetic device, a microprocessor, amicrocomputer, a processor, or a digital signal processor (DSP).

FIG. 4 is a hardware block diagram of the computer in the case in whichthe digital signal processor 14 is implemented by software, firmware, orthe like.

In the case in which the digital signal processor 14 is implemented bysoftware, firmware, or the like, a program for causing the computer toperform processing procedures of the adaptive equalizer 21, the errorcorrector 22, the error ratio calculator 23, the margin calculator 24,and the tap number controller 25 is stored in a memory 51. Then, aprocessor 52 of the computer executes the program stored in the memory51.

FIG. 5 is a flowchart showing a part of a light signal receiving methodwhich is the processing procedures in the case in which the digitalsignal processor 14 is implemented by software, firmware, or the like.

Further, in FIG. 3, the example in which each of the components of thedigital signal processor 14 is implemented by hardware for exclusive useis shown, and in FIG. 4, the example in which the digital signalprocessor 14 is implemented by software, firmware, or the like is shown.However, these are only examples, and some of the components of thedigital signal processor 14 may be implemented by hardware for exclusiveuse and the remaining components may be implemented by software,firmware, or the like.

Next, the operation of the data reproduction device shown in FIG. 1 willbe explained.

The light source 11 generates local oscillation light having the samewavelength as the polarization multiplex light into which the horizontalpolarization signal and the vertical polarization signal aremultiplexed, and outputs the local oscillation light to the coherentreceiver 12.

The coherent receiver 12 receives the polarization multiplex light, andperforms coherent detection on the polarization multiplex light by usingthe local oscillation light outputted from the light source 11, to splitthe polarization multiplex light into the horizontally polarized lightand the vertically polarized light.

The coherent receiver 12 converts the horizontally polarized light intoan electric signal, and outputs the electric signal, as a horizontalpolarization signal, to the analog to digital converter 13.

The coherent receiver 12 converts the vertically polarized light into anelectric signal, and outputs the electric signal, as a verticalpolarization signal, to the analog to digital converter 13.

When receiving the horizontal polarization signal from the coherentreceiver 12, the analog to digital converter 13 converts the horizontalpolarization signal from an analog signal into a digital signal, andoutputs this digital signal as a digital horizontal polarization signalto the digital signal processor 14.

When receiving the vertical polarization signal from the coherentreceiver 12, the analog to digital converter 13 converts the verticalpolarization signal from an analog signal into a digital signal, andoutputs this digital signal as a digital vertical polarization signal tothe digital signal processor 14.

When receiving the digital horizontal polarization signal from theanalog to digital converter 13, the adaptive equalizer 21 of the digitalsignal processor 14 compensates for the waveform distortion of the firsttransmission signal superimposed onto the digital horizontalpolarization signal by using the first FIR filter (step ST1 of FIG. 5).

When receiving the digital vertical polarization signal from the analogto digital converter 13, the adaptive equalizer 21 compensates for thewaveform distortion of the second transmission signal superimposed ontothe digital vertical polarization signal by using the second FIR filter(step ST1 of FIG. 5).

As the compensation for the waveform distortion in the adaptiveequalizer 21, linear compensation for the waveform distortion, such asfrequency offset compensation or wavelength dispersion compensation, isprovided.

Further, the adaptive equalizer 21 generates first decoded data bydecoding the first transmission signal after the waveform distortioncompensation, and outputs the first decoded data to the error corrector22 (step ST2 of FIG. 5).

The adaptive equalizer 21 generates second decoded data by decoding thesecond transmission signal after the waveform distortion compensation,and outputs the second decoded data to the error corrector 22 (step ST2of FIG. 5).

As the process of decoding each of the first and second transmissionsignals in the adaptive equalizer 21, a symbol determination processshown below can be considered, for example.

The adaptive equalizer 21 performs the symbol determination process ofcomparing each of the transmission signals and a threshold, andassigning data of “1” as decoded data when the transmission signal isgreater than the threshold, whereas assigning data of “0” as decodeddata when the transmission signal is equal to or less than thethreshold.

It is assumed that the threshold is stored in an internal memory of theadaptive equalizer 21. The threshold may be provided for the adaptiveequalizer 21 from the outside.

When receiving the first decoded data from the adaptive equalizer 21,the error corrector 22 performs error correction to the first decodeddata and calculates the number of error corrections in the first decodeddata (step ST3 of FIG. 5).

When receiving the second decoded data from the adaptive equalizer 21,the error corrector 22 performs error correction to the second decodeddata and calculates the number of error corrections in the seconddecoded data (step ST3 of FIG. 5).

As a method of performing error correction to decoded data, a method ofcorrecting errors in the decoded data by using a low density paritycheck (LDPC) code can be used, for example.

The error corrector 22 outputs each of the first and second pieces ofdecoded data after the error correction to the data reproducer 30.

The error corrector 22 outputs each of the numbers of error correctionsin the first and second pieces of decoded data to the error ratiocalculator 23.

When receiving the number of error corrections in the first decoded datafrom the error corrector 22, the error ratio calculator 23 calculatesthe bit error ratio of the first decoded data by dividing the number oferror corrections in the first decoded data by the total number of bitsof the first decoded data (step ST4 of FIG. 5).

When receiving the number of error corrections in the second decodeddata from the error corrector 22, the error ratio calculator 23calculates the bit error ratio of the second decoded data by dividingthe number of error corrections in the second decoded data by the totalnumber of bits of the second decoded data (step ST4 of FIG. 5).

The error ratio calculator 23 outputs each of the bit error ratios ofthe first and second pieces of decoded data to the margin calculator 24.

When receiving the bit error ratio of the first decoded data from theerror ratio calculator 23, the margin calculator 24 calculates a firstmargin from the bit error ratio of the error correction limit in the biterror ratio of the first decoded data, as shown in the followingequation (1) (step ST5 of FIG. 5).First margin=bit error ratio of error correction limit−bit error ratioof first decoded data  (1)

When receiving the bit error ratio of the second decoded data from theerror ratio calculator 23, the margin calculator 24 calculates a secondmargin from the bit error ratio of the error correction limit in the biterror ratio of the second decoded data, as shown in the followingequation (2) (step ST5 of FIG. 5).Second margin=bit error ratio of error correction limit−bit error ratioof second decoded data  (2)

The bit error ratio of the error correction limit is the maximum errorratio at which bit errors can be corrected by the error corrector 22,and is a value which is determined depending on the configuration forperforming error correction, and so on. The bit error ratio of the errorcorrection limit is known in the margin calculator 24.

The margin calculator 24 outputs each of the first and second margins tothe tap number controller 25.

When receiving the first margin from the margin calculator 24, the tapnumber controller 25 sets up the number of taps of the first FIR filterwhich the adaptive equalizer 21 uses, on the basis of the first margin(step ST6 of FIG. 5).

When receiving the second margin from the margin calculator 24, the tapnumber controller 25 sets up the number of taps of the second FIR filterwhich the adaptive equalizer 21 uses, on the basis of the second margin(step ST6 of FIG. 5).

FIG. 6 is a flowchart showing the process of setting up the numbers oftaps in the tap number controller 25.

Hereinafter, the process of setting up the numbers of taps in the tapnumber controller 25 will be explained concretely with reference to FIG.6.

It is assumed that a threshold Th for determining the size of eachmargin is stored in an internal memory of the tap number controller 25.The threshold Th may be provided for the tap number controller 25 fromthe outside.

Further, it is assumed that in the internal memory of the tap numbercontroller 25, “Tc1” is stored as a first tap number and “Tc2” is storedas a second tap number. The following relationship: Tc1<Tc2 isestablished. The first and second tap numbers may be provided for thetap number controller 25 from the outside.

When receiving the first margin from the margin calculator 24, the tapnumber controller 25 compares the first margin and the threshold Th(step ST11 of FIG. 6).

When the first margin is greater than the threshold Th (when Yes in stepST11 of FIG. 6), the tap number controller 25 sets the number taps ofthe first FIR filter which the adaptive equalizer 21 uses to “Tc1” (stepST12 of FIG. 6).

Because the performance of resisting variations in the horizontallypolarized wave is improved by setting the number of taps of the firstFIR filter to “Tc1” less than “Tc2”, the influence of a variation in thehorizontally polarized wave can be reduced even when the variation inthe horizontally polarized wave is a rapid one varying in a time ofseveral tens of microseconds.

When the first margin is equal to or less than the threshold Th (when Noin step ST11 of FIG. 6), the tap number controller 25 sets the number oftaps of the first FIR filter which the adaptive equalizer 21 uses to“Tc2” (step ST13 of FIG. 6).

By setting the number of taps of the first FIR filter to “Tc2” greaterthan “Tc1”, the performance of improving the bit error ratio can beimproved. There is a trade-off relation between the performance ofresisting variations in the horizontally polarized wave and theperformance of improving the bit error ratio.

When receiving the second margin from the margin calculator 24, the tapnumber controller 25 compares the second margin and the threshold Th(step ST11 of FIG. 6).

When the second margin is greater than the threshold Th (when Yes instep ST11 of FIG. 6), the tap number controller 25 sets the number tapsof the second FIR filter which the adaptive equalizer 21 uses to “Tc1”(step ST12 of FIG. 6).

Because the performance of resisting variations in the verticallypolarized wave is improved by setting the number of taps of the secondFIR filter to “Tc1” less than “Tc2”, the influence of a variation in thevertically polarized wave can be reduced even when the variation in thevertically polarized wave is a rapid one varying in a time of severaltens of microseconds.

When the second margin is equal to or less than the threshold Th (whenNo in step ST11 of FIG. 6), the tap number controller 25 sets the numberof taps of the second FIR filter which the adaptive equalizer 21 uses to“Tc2” (step ST13 of FIG. 6).

By setting the number of taps of the second FIR filter to “Tc2” greaterthan “Tc1”, the performance of improving the bit error ratio can beimproved. There is a trade-off relation between the performance ofresisting variations in the vertically polarized wave and theperformance of improving the bit error ratio.

When receiving the first decoded data after the error correction and thesecond decoded data after the error correction from the error corrector22, the data reproducer 30 records data of “00” when the first decodeddata is data of “0” and the second decoded data is data of “0.”

The data reproducer 30 records data of “10” when the first decoded datais data of “1” and the second decoded data is data of “0.”

The data reproducer 30 records data of “01” when the first decoded datais data of “0” and the second decoded data is data of “1.”

The data reproducer 30 records data of “11” when the first decoded datais data of “1” and the second decoded data is data of “1.”

By reproducing the recorded data, the data reproducer 30 draws an image,for example.

Here, FIG. 7 is an explanatory drawing showing a relation between thenumber of taps of each FIR filter, and the margin in the bit error ratioof the decoded data.

In FIG. 7, the horizontal axis shows the number of taps of each FIRfilter, and the vertical axis shows the margin in the bit error ratio ofthe decoded data.

In FIG. 7, an experimental result when no polarization variation occurs(when the polarization variation is 0 kHz), and an experimental resultwhen a high-speed polarization variation occurs (when the polarizationvariation is 150 kHz) are shown.

⋄ shows an instant margin when the polarization variation is 0 kHz, and□ shows an average margin when the polarization variation is 0 kHz.

Δ shows an instant margin when the polarization variation is 150 kHz,and ◯ shows an average margin when the polarization variation is 150kHz.

(1) shows changes of the margin when a high-speed polarization variationoccurs in the case in which although the step size parameter isadaptively controlled using the method described in Patent Literature 1,the number of taps of each FIR filter is constant. In FIG. 7, the numberof taps of each FIR filter is fixed to 17.

(2) shows changes of the margin when a high-speed polarization variationoccurs in the case in which the number of taps of each FIR filter is setup by the tap number controller 25.

In FIG. 7, the number of taps of each FIR filter is set to Tc1=11because the margin is greater than the threshold Th.

In the case of using the method described in Patent Literature 1,because both the instant margin and the average margin are greater than0.0, as shown by (1), when the polarization variation is 0 kHz, theredoes not occur degradation in the transmission quality which is causedby the polarization variation, and there is a low possibility that anoptical communication interruption is caused.

In the case of using the method described in Patent Literature 1,because the instant margin is less than 0.0, as shown by (1), when thepolarization variation is 150 kHz, the degradation in the transmissionquality which is caused by the polarization variation is large, andthere is a high possibility that an optical communication interruptionis caused.

In the case in which the number of taps of each FIR filter is set up bythe tap number controller 25, because both the instant margin and theaverage margin are greater than 0.0, as shown by (2), when thepolarization variation is 0 kHz, there does not occur degradation in thetransmission quality which is caused by the polarization variation, andthere is a low possibility that an optical communication interruption iscaused.

Even in the case in which the number of taps of each FIR filter is setup by the tap number controller 25, both the instant margin and theaverage margin are reduced, as shown by (2), when the polarizationvariation is 150 kHz.

However, because both the instant margin and the average margin aregreater than 0.0, as shown by (2), the degradation in the transmissionquality which is caused by the polarization variation is small, andthere is a low possibility that an optical communication interruption iscaused.

FIG. 8 is an explanatory drawing showing a relation between the numberof taps of each FIR filter, and the margin in the bit error ratio of thedecoded data.

In FIG. 8, the horizontal axis shows the number of taps of each FIRfilter, and the vertical axis shows the margin in the bit error ratio ofthe decoded data.

In FIG. 8, an experimental result when no polarization variation occurs(when the polarization variation is 0 kHz), and an experimental resultwhen a high-speed polarization variation occurs (when the polarizationvariation is 150 kHz) are shown.

⋄, □, Δ, and ◯ show margins, like those of FIG. 7.

The margins shown in FIG. 8 are less than those shown in FIG. 7.

In the case of using the method described in Patent Literature 1, alsoin FIG. 8, the number of taps of each FIR filter is fixed to 17, like inFIG. 7.

In FIG. 8, because the margin is equal to or less than the threshold Th,the number of taps of each FIR filter is set to Tc2=17 by the tap numbercontroller 25.

Even in the case in which the number of taps of each FIR filter is setup by the tap number controller 25, and even in the case of using themethod described in Patent Literature 1, both the instant margin and theaverage margin are less than 0.0 when the polarization variation is 150kHz. Therefore, the degradation in the transmission quality which iscaused by the polarization variation is large, and there is a highpossibility that an optical communication interruption is caused.

Even if the number of taps of each FIR filter is set to Tc1=11 by thetap number controller 25, because the margin when the polarizationvariation is 0 kHz is small, both the instant margin and the averagemargin are less than 0.0 when a high-speed polarization variationoccurs. Therefore, a high priority is given to an improvement of theperformance of improving the bit error ratio, so that the number of tapsof each FIR filter is set to Tc2=17.

Even in the case in which the number of taps of each FIR filter is setup by the tap number controller 25, and even in the case of using themethod described in Patent Literature 1, both the instant margin and theaverage margin are greater than 0.0 when the polarization variation is 0kHz. Therefore, the degradation in the transmission quality which iscaused by the polarization variation is small, and there is a lowpossibility that an optical communication interruption is caused.

In above-mentioned Embodiment 1, the optical receiver 1 is configured insuch a way as to include the tap number controller 25 for setting up thenumbers of taps of the first and second FIR filters on the basis of therespective margins calculated by the margin calculator 24. Therefore,the optical receiver 1 can suppress the degradation in the transmissionquality which is caused by the influence of the polarization variationeven in the case in which the polarization variation is a rapid onevarying in a time of several tens of microseconds.

Embodiment 2

In the optical receiver 1 of Embodiment 1, the example in which theerror ratio calculator 23 calculates the bit error ratios of the firstand second decoded data is shown.

In Embodiment 2, an error ratio calculator 23 repeatedly calculates thebit error ratio of first decoded data multiple times, and compares themultiple calculated bit error ratios to determine a bit error ratio tobe outputted to a margin calculator 24. An optical receiver 1 in whichthe error ratio calculator 23 also calculates repeatedly the bit errorratio of second decoded data multiple times, and compares the multiplecalculated bit error ratios to determine a bit error ratio to beoutputted to the margin calculator 24 will be explained.

Hereinafter, the process of calculating bit error ratios in the errorratio calculator 23 will be explained concretely.

A calculation time period which is a time period during which to performthe process of calculating bit error ratios is stored in an internalmemory of the error ratio calculator 23. For example, the calculationtime period is 1 minute.

Further, the length of calculation time intervals at which a bit errorratio is repeatedly calculated is stored in the internal memory of theerror ratio calculator 23. For example, the length of calculation timeintervals is 1 millisecond.

The calculation time period and the length of calculation time intervalsmay be provided for the error ratio calculator 23 from the outside.

The error ratio calculator 23 repeatedly calculates the bit error ratioof the first decoded data at calculation time intervals during thecalculation time period.

The error ratio calculator 23 compares the multiple bit error ratios ofthe first decoded data which have been repeatedly calculated during thecalculation time period.

The error ratio calculator 23 selects the largest bit error ratio out ofthe multiple bit error ratios of the first decoded data, and outputs theselected bit error ratio to the margin calculator 24.

Further, the error ratio calculator 23 repeatedly calculates the biterror ratio of the second decoded data at calculation time intervalsduring the calculation time period.

The error ratio calculator 23 compares the multiple bit error ratios ofthe second decoded data which have been repeatedly calculated during thecalculation time period.

The error ratio calculator 23 selects the largest bit error ratio out ofthe multiple bit error ratios of the second decoded data, and outputsthe selected bit error ratio to the margin calculator 24.

In above-mentioned Embodiment 2, the error ratio calculator 23repeatedly calculates the bit error ratio of each decoded data multipletimes, and compares the multiple calculated bit error ratios todetermine each bit error ratio to be outputted to the margin calculator24.

Therefore, when the length of time intervals at which the bit errorratio is repeatedly calculated is shorter than the variation time of thepolarization variation, the bit error ratio of each decoded data, thebit error ratio being adapted to the polarization variation, can becalculated.

It is to be understood that a combination of the above-mentionedembodiments can be made, various changes can be made in any componentaccording to any one of the above-mentioned embodiments, or anycomponent according to any one of the above-mentioned embodiments can beomitted within the scope of the present disclosure.

INDUSTRIAL APPLICABILITY

The present disclosure is suitable for an optical receiver, a lightsignal receiving method, and a data reproduction device that set up thenumber of taps of a finite impulse response filter.

REFERENCE SIGNS LIST

1 optical receiver, 11 light source, 12 coherent receiver, 13 analog todigital converter, 14 digital signal processor, 21 adaptive equalizer,22 error corrector, 23 error ratio calculator, 24 margin calculator, 25tap number controller, 30 data reproducer, 41 adaptive equalizationcircuit, 42 error correction circuit, 43 error ratio calculationcircuit, 44 margin calculation circuit, 45 tap number setting circuit,51 memory, and 52 processor.

What is claimed is:
 1. An optical receiver comprising: a coherentreceiver performing coherent detection on polarization multiplex lightinto which first polarization light and second polarization light aremultiplexed, and splitting the polarization multiplex light into thefirst polarization light and the second polarization light; an adaptiveequalizer compensating waveform distortion of a signal superimposed ontothe first polarization light by using a first finite impulse responsefilter, compensating waveform distortion of a signal superimposed ontothe second polarization light by using a second finite impulse responsefilter, and by decoding each of the signals whose waveform distortionhas been compensated, generating their respective decoded data; an errorratio calculator calculating a bit error ratio of each decoded datagenerated by the adaptive equalizer; a margin calculator calculating amargin from a bit error ratio of an error correction limit in each biterror ratio calculated by the error ratio calculator; and a tap numbercontroller setting up numbers of taps of the first and second finiteimpulse response filters by referring to respective margins calculatedby the margin calculator.
 2. The optical receiver according to claim 1,wherein the optical receiver comprises an error corrector performingerror correction to each decoded data generated by the adaptiveequalizer, and calculating a number of error corrections in each decodeddata, and wherein the error ratio calculator calculates the bit errorratio of each decoded data from the number of error corrections in thedecoded data, the number being calculated by the error corrector.
 3. Theoptical receiver according to claim 1, wherein the error ratiocalculator repeatedly calculates the bit error ratio of each decodeddata multiple times, compares the multiple calculated bit error ratiosfor each decoded data, and determines one among the calculated bit errorratios to be outputted to the margin calculator for each decoded data.4. The optical receiver according to claim 1, wherein the tap numbercontroller sets the numbers of taps of the first and second finiteimpulse response filters to a first tap number when each margincalculated by the margin calculator is greater than a threshold, whereasthe tap number controller sets the number of taps of the first andsecond finite impulse response filters to a second tap number largerthan the first tap number when each margin is equal to or less than thethreshold.
 5. A light signal receiving method comprising the steps of:performing coherent detection by a coherent receiver, on polarizationmultiplex light into which first polarization light and secondpolarization light are multiplexed, and splitting the polarizationmultiplex light into the first polarization light and the secondpolarization light; compensating waveform distortion by an adaptiveequalizer, of a signal superimposed onto the first polarization light byusing a first finite impulse response filter, compensating waveformdistortion of a signal superimposed onto the second polarization lightby using a second finite impulse response filter, and by decoding eachof the signals whose waveform distortion has been compensated,generating their respective decoded data; calculating a bit error ratioby an error ratio calculator, of each decoded data generated by theadaptive equalizer; calculating a margin by a margin calculator, from abit error ratio of an error correction limit in each bit error ratiocalculated by the error ratio calculator; and setting up number of tapsby a tap number controller, of the first and second finite impulseresponse filters by referring to respective margins calculated by themargin calculator.
 6. A data reproduction device comprising: a coherentreceiver for performing coherent detection on polarization multiplexlight into which first polarization light and second polarization lightare multiplexed, and splitting the polarization multiplex light into thefirst polarization light and the second polarization light; an adaptiveequalizer compensating waveform distortion of a signal superimposed ontothe first polarization light by using a first finite impulse responsefilter, compensating waveform distortion of a signal superimposed ontothe second polarization light by using a second finite impulse responsefilter, by decoding each of the signals whose waveform distortion hasbeen compensated, generating their respective decoded data; an errorratio calculator calculating a bit error ratio of each decoded datagenerated by the adaptive equalizer; a margin calculator calculating amargin from a bit error ratio of an error correction limit in each biterror ratio calculated by the error ratio calculator; a tap numbercontroller setting up numbers of taps of the first and second finiteimpulse response filters by referring to respective margins calculatedby the margin calculator; and a data reproducer reproducing each decodeddata generated by the adaptive equalizer.